This invention relates generally to an internal combustion engine capable of operating in, and transitioning between, different operating modes including a premixed charge compression ignition mode, a diesel mode and/or spark ignition mode.
For well over 75 years the internal combustion engine has been mankind""s primary source of motive power. It would be difficult to overstate its importance or the engineering effort expended in seeking its perfection. So mature and well understood is the art of internal combustion engine design that most so called xe2x80x9cnewxe2x80x9d engine designs are merely designs made up of choices among a variety of known alternatives. For example, an improved output torque curve can easily be achieved by sacrificing engine fuel economy. Emissions abatement or improved reliability can also be achieved with an increase in cost. Still other objectives can be achieved such as increased power and reduced size and/or weight but normally at a sacrifice of both fuel efficiency and low cost.
The challenge to contemporary designers has been significantly increased by the need to respond to governmentally mandated emissions abatement standards while maintaining or improving fuel efficiency. In view of the mature nature of engine design, it is extremely difficult to extract both improved engine performance and emissions abatement from further innovations of the basic engine designs commercially available today. Yet the need for such innovations has never been greater in view of the series of escalating emissions standards mandated for the future by the United States government and other countries. Attempts to meet these standards include some designers looking for a completely new engine design.
Traditionally, there have been two primary forms of reciprocating piston or rotary internal combustion engines: diesel and spark ignition engines. While these engine types have similar architecture and mechanical workings, each has distinct operating properties which are vastly different from each other. Diesel and spark ignited engines effectively control the start of combustion (SOC) using simple, yet distinct means. The diesel engine controls the SOC by the timing of fuel injection. In a spark ignited engine, the SOC is controlled by the spark timing. As a result, there are important differences in the advantages and disadvantages of diesel and spark-ignited engines. The major advantage that a spark-ignited natural gas, or gasoline, engine has over a diesel engine is the ability to achieve extremely low NOx and particulate emissions levels. The major advantage that diesel engines have over premixed charge spark ignited engines (such as passenger car gasoline engines and lean burn natural gas engines) is higher thermal efficiency. One key reason for the higher efficiency of diesel engines is the ability to use higher compression ratios than premixed charge spark ignited engines (the compression ratio in premixed charge spark ignited engines has to be kept relatively low to avoid knock). A second key reason for the higher efficiency of diesel engines lies in the ability to control the diesel engine""s power output without a throttle. This eliminates the throttling losses of premixed charge spark ignited engines and results in significantly higher efficiency at part load for diesel engines. Typical diesel engines, however, cannot achieve the very low NOx and particulate emissions levels which are possible with premixed charge spark ignited engines. Due to the mixing controlled nature of diesel combustion a large fraction of the fuel exists at a very fuel rich equivalence ratio which is known to lead to particulate emissions. Premixed charge spark ignited engines, on the other hand, have nearly homogeneous air fuel mixtures which tend to be either lean or close to stoichiometric, resulting in very low particulate emissions. Another consideration is that the mixing controlled combustion in diesel engines occurs when the fuel and air exist at a near stoichiometric equivalence ratio which leads to high temperatures. The high temperatures, in turn, cause high NOx emissions. Lean burn premixed charge spark ignited engines, on the other hand, bum their fuel at much leaner equivalence ratios which results in significantly lower temperatures leading to much lower NOx emissions. Stoichiometric premixed charge spark ignited engines, on the other hand, have high NOx emissions due to the high flame temperatures resulting from stoichiometric combustion. However, the virtually oxygen free exhaust allows the NOx emissions to be reduced to very low levels with a three-way catalyst.
Relatively recently, some engine designers have directed their efforts to another type of engine which utilizes premixed charge compression ignition (PCCI) or homogeneous charge compression ignition (HCCI), hereinafter collectively referred to as PCCI. Engines operating on PCCI principles rely on autoignition of a relatively well premixed fuel/air mixture to initiate combustion. Importantly, the fuel and air are mixed upstream of the cylinder, e.g., in the intake port, or in the cylinder, long before ignition occurs. The extent of the mixture may be varied depending on the combustion characteristics desired. Some engines are designed and/or operated to ensure the fuel and air are mixed into a homogeneous, or nearly homogeneous, state. Also, an engine may be specifically designed and/or operated to create a somewhat less homogeneous charge having a small degree of stratification. In both instances, the mixture exists in a premixed state well before ignition occurs and is compressed until the mixture autoignites. Thus, PCCI combustion is characterized in that: 1) the vast majority of the fuel is sufficiently premixed with the air to form a combustible mixture throughout the charge by the time of ignition; and 2) ignition, that is, the very onset or start of combustion, is initiated by compression ignition. Unlike a diesel engine, the timing of the fuel delivery, for example the timing of injection, in a PCCI engine does not strongly affect the timing of ignition. Preferably, PCCI combustion is characterized in that most of the mixture is significantly leaner than stoichiometric to advantageously reduce emissions, unlike the typical diesel engine cycle in which a large portion, or all, of the mixture exists in a rich state during combustion.
Because an engine operating on PCCI combustion principles has the potential for providing the excellent fuel economy of the diesel engine while providing NOx and particulate emissions levels that are much lower than that of current spark-ignited engine, it has also recently been the subject of extensive research and development. U.S. Pat. Nos. 4,768,481; 5,535,716; and 5,832,880 all disclose engines and methods for controlling PCCI combustion in engines. Researchers have used various other names in referencing PCCI combustion including homogeneous charge compression ignition (HCCI) as well as others such as xe2x80x9cATACxe2x80x9d which stands for xe2x80x9cActive Thermo-Atmosphere Combustion.xe2x80x9d (SAE Technical Paper No. 790501, Feb. 26-Mar. 2, 1979), xe2x80x9cTSxe2x80x9d which stands for xe2x80x9cToyota-Sokenxe2x80x9d (SAE Technical Paper No. 790840, Sep. 10-13, 1979), and xe2x80x9cCIHCxe2x80x9d which stands for xe2x80x9ccompression-ignited homogeneous chargexe2x80x9d (SAE Paper No. 830264, 1983). All of these terms are hereinafter collectively referred to as PCCI.
Although PCCI combustion may result in improved fuel economy and substantially reduced emissions, it is difficult for an engine to operate in a PCCI mode over a wide range of operating conditions, ranging from cold start-up to various levels of engine load. For example, SAE Technical Paper No. 790501 reports that PCCI combustion (ATAC) could be made to occur in a two-stroke engine at low load over a wide speed range. To attain PCCI combustion, the following conditions were found to be important. The quantity of mixture and the air/fuel ratio supplied to the cylinder must be uniform from cycle to cycle. The scavenging xe2x80x9cdirectivityxe2x80x9d and velocity must have cyclic regularity to ensure the correct condition of the residual gases remaining in the cylinder. The temperature of the combustion chamber walls must be suitable. The scavenging passage inlet must be located at the bottom of the crankcase. It was found that at very light loads, PCCI was not successful because charge temperatures were too low. At very high loads, PCCI was not successful because the residual gas quantity was too low. In between these regions, PCCI combustion was successful.
As a result, research has been directed to an engine capable of operating in multiple combustion modes. For example, SAE Technical Paper No. 892068, entitled xe2x80x9cHomogeneous-Charge Compression Ignition (HCCI) Enginesxe2x80x9d, Thring, R., Sep. 25, 1989, investigated PCCI operation of a four-stroke engine. The paper suggests an engine that would operate in a conventional spark-ignition mode at start-up and at high loads, but in a PCCI mode at part-load and idle. Others have produced two-stroke motorcycle engines which successfully use a spark to initiate combustion upon starting the engine, at the lowest load conditions, such as idling, and at high loads while operating in a PCCI mode during a low to mid-load range. The change-over between spark-ignition and PCCI modes is controlled by an electronic control unit. SAE papers 920512 and 972874 are noted for disclosing experimental results comparing PCCI combustion to spark-ignition combustion, but fail to specifically teach the manner in which transitions between modes of operation could be most effectively achieved. German Patent No. 198 18 596 also discloses a process of operating an engine in a PCCI mode at least low loads and in a spark-ignition mode at high loads.
Other efforts have focused on the combination of a diesel combustion operating mode and a PCCI mode. For example, SAE paper No. 971676 entitled xe2x80x9cHomogeneous Charge Compression Ignition (HCCI) of Diesel Fuelxe2x80x9d reports test results of an engine which includes starting in a diesel combustion mode and, once the temperature of the engine stabilized, configuring the engine to a PCCI mode. U.S. Pat. No. 5,875,743 discloses an engine which operates in a diesel combustion mode in response to engine operating parameters indicative of engine speed and load values within a first predefined range, and in a PCCI mode in response to engine operating parameters indicative of engine speed and load within a second predefined range. Generally, the engine appears to operate in a diesel mode during light and heavy loads and in a PCCI mode at other conditions. A look-up table may be used to define the speed and load ranges at which the engine will run in conventional diesel mode, and the speeds and load ranges the engine will switch to the PCCI mode. If misfire or knock is detected, the PCCI mode can be adjusted or the engine switched back to the diesel mode. Transition between the diesel mode to the PCCI mode is primarily accomplished by switching between an in-cylinder fuel injector and a port injector for early injection and mixing of fuel, or varying the timing of injection of the in-cylinder injector.
Patent application Ser. No. 08/916,437 filed on Aug. 22, 1997 (published as International Patent Application No. PCT/US97/14815), currently assigned to the Assignee of the present invention, discloses an engine and method of operation which includes multiple combustion modes. The engine is switched between a conventional diesel mode and/or spark-ignited mode and a PCCI mode depending on the operating conditions of the engine.
Still, there is a need for an engine, and method of engine operation, which includes more effectively and more efficiently operating in, and transitioning between, a PCCI mode and one or both of a diesel mode and a spark-ignition mode.
A general object of the subject invention is to overcome the deficiencies of the prior art by providing a practical multi-mode engine and a method for operating the engine in various modes and effectively and efficiently transferring operation between the various modes.
Another object of the present invention is to provide a multi-mode internal combustion engine having minimum complexity and maximum robustness while maximizing efficiency.
Yet another object of the present invention is to provide a multi-mode internal combustion engine capable of transitioning between a diesel mode while maximizing natural gas usage and minimizing diesel flow rates.
Still another object of the present invention is to provide a multi-mode engine and control system which achieves a higher gas substitution rate thereby reducing operating costs due to the lower costs of natural gas per BTU.
A further object of the present invention is to provide a multi-mode engine capable of smoothly transitioning between various modes of operation/combustion in a manner to obtain a smooth and controlled power delivery and sound quality from the engine.
A still further object of the present invention is to provide a multi-mode engine and control system capable of effectively placing the engine in a homogeneous charge dual fuel transition mode for transitioning between the various modes of operation.
A still further object of the present invention is to provide a multi-mode engine and control scheme for controlling the engine in a manner to optimally minimize emissions, especially oxides of nitrogen and particulate emissions, while maximizing efficiency.
Still another object of the present invention is to provide a multi-mode engine which permits a cold engine to be more easily started and then transitioned to one or more other modes based on engine operating conditions.
A further object of the present invention is to provide a multi-mode engine capable of operating on a single fuel throughout various combustion modes.
A still further object of the present invention is to provide a multi-mode engine capable of effectively transferring operation between a spark ignited mode and a PCCI mode by rapidly changing the equivalence ratio while maintaining the engine torque essentially constant.
A further object of the present invention is to provide a multi-mode engine capable of transferring engine operation between operating modes while avoiding very heavy, destructive knock, misfire, carbon monoxide emissions and/or undesirable levels of unburned hydrocarbons.
Yet another object of the present invention is to provide a multi-mode engine and control system which effectively control characteristics of the combustion event, such as the timing of the start of combustion, during the various modes and during transfer between the various modes to ensure stable combustion, low emissions, acceptable pressure levels and optimum efficiency.
The above objects and others are achieved by providing a multi-mode internal combustion engine capable of operating in a plurality of modes for engine operation, comprising an engine body, a combustion chamber formed in the engine body, an intake air system for delivering intake air to the combustion chamber, a fuel delivery system mounted on the engine body to deliver a first fuel into the combustion chamber while the engine operates in a diesel mode and a homogeneous charge dual fuel transition mode, and to deliver a second fuel into at least one of the intake air system and the combustion chamber when the engine operates in a premixed charge compression ignition mode and in the homogeneous charge dual fuel transition mode. The engine also includes a control system adapted to transfer engine operation between the diesel mode and the homogeneous charge dual fuel transition mode and between the homogeneous charge dual fuel transition mode and the premixed charge compression ignition mode.
The control system may be adapted to cause the fuel delivery system to deliver a primary quantity of the first fuel into the combustion chamber when in the diesel mode and, when transferring engine operation to the homogeneous charge dual fuel transition mode, to cause the fuel delivery system to deliver a quantity of the second fuel into at least one of the intake air system and the combustion chamber while decreasing the primary quantity of the first fuel to maintain engine torque at a substantially constant level and to place the engine in the homogeneous charge dual fuel transition mode. The control system may further be adapted to cause the fuel delivery system to decrease the primary quantity of the first fuel while increasing the quantity of the second fuel so that the quantity of the second fuel comprises a substantial portion of the total delivered fuel energy. The control system may further be adapted to control a start of combustion in the combustion chamber and adjust the start of combustion to occur prior to delivery of the primary quantity of the first fuel. The control system may further be adapted to cause the fuel delivery system to deliver an early pilot quantity of the first fuel prior to a combustion in the combustion chamber when in the homogeneous charge dual fuel transition mode. The control system may be further adapted to cause the fuel delivery system to increase the early diesel pilot quantity of the first fuel sufficiently to cause the start of combustion to occur prior to the delivery of the primary quantity of the first fuel. Moreover, the control system may cause the fuel delivery system to deliver a post-ignition injection of the first fuel into the combustion chamber after a start of combustion of a premixed charge of the second fuel and air in the combustion chamber when in the premixed charge compression ignition mode to operate the engine in a post premixed ignition mode. The control system may also cause the fuel delivery system to deliver an early pilot quantity of the first fuel prior to a start of combustion of a premixed charge of the second fuel and air in the combustion chamber when in the premixed charge compression ignition mode. The first fuel may be one of diesel fuel, kerosene and gasoline and the second fuel may be one of natural gas and propane. The present invention is also directed to a method of operating an internal combustion engine in the plurality of modes and transferring operation between the plurality of modes which includes operating the engine in the diesel mode, operating the engine in the premixed charge compression ignition mode and operating the engine in a homogeneous charge dual fuel transition mode when transferring engine operation between the diesel mode and the premixed charge compression ignition mode. The engine may use a single fuel for all modes of operation and the single fuel may be diesel fuel or gasoline. The amount of the single fuel provided to a combustion chamber may be adjusted to adjust a timing of a start of combustion. Also, the timing of an opening of an intake valve associated with the combustion chamber may be controlled to vary an effective compression ratio to control a start of combustion. An exhaust gas may also be directed into the combustion chamber to control a start of combustion. The method may include the step of sensing a combustion characteristic, generating a combustion characteristic signal and controlling a start of combustion based on the combustion characteristic signal. The method may also include operating the engine in a spark ignition mode and operating the engine in a homogeneous charge dual fuel transition mode when transferring engine operation between the premixed charge compression ignition mode and the spark ignition mode. The spark ignition mode may include a liquid spark comprising a pilot quantity of fuel for igniting a premixed charge of fuel and air.
The objects are also achieved by providing a multi-mode internal combustion engine capable of operating in a plurality of modes wherein the control system is adapted to transfer engine operation between a spark ignition mode and a homogeneous charge dual fuel transition mode and between the homogeneous charge dual fuel transition mode and the premixed charge compression ignition mode. The method associated with transferring operation between a spark ignition mode and a premixed charge compression ignition mode via the homogeneous charge dual fuel transition mode may also include a step of providing intake air and a second fuel to the combustion chamber and providing a throttle in the intake system for controlling the intake flow of at least one of the intake air and a premixed charge of intake air and the second fuel. The method may further include the step of operating the engine in the spark ignition mode with throttle valve partially closed to restrict the intake flow into the combustion chamber wherein the premixed charge of the second fuel and air has an equivalence ratio greater than 0.5. The method may also include the step of decreasing a quantity of the second fuel in the premixed charge while increasing a quantity of a first fuel delivered into the combustion chamber in a manner to maintain engine torque at a substantially constant level and to reduce the equivalence ratio of the premixed charge to less than 0.5. The method would then open the throttle valve to increase intake flow and terminate the flow of the first fuel into the combustion chamber to transfer the engine to the premixed charge compression ignition mode. This method may include opening the throttle valve and terminating the flow of the first fuel nearly simultaneously while maintaining the total delivered fuel energy at a substantially constant level. Moreover the method may include the step of opening the throttle valve to increase the intake flow in a manner which approximately doubles a total amount of second fuel delivered to the combustion chamber. The method may also decrease the quantity of the second fuel while increasing the quantity of the first fuel until the second fuel and the first fuel each contribute approximately 50% of the total delivered fuel energy.
The present invention is also directed to a method of operating an internal combustion engine in a plurality of modes for engine operation and transferring operation between the modes which includes delivering fuel into one of the intake port and the combustion chamber at a predetermined flow rate, operating the engine in a spark ignition mode with the throttle valve partially closed to restrict intake air flow to the intake port wherein the fuel and the intake air form a premixed charge having an equivalence ratio greater than 0.5. This method includes the step of opening the throttle valve to increase the intake air flow while maintaining the predetermined fuel rate substantially constant to reduce the equivalence ratio of the premixed charge to less than 0.5 to place the engine a premixed charge compression ignition mode. The present invention is also directed to a multi-mode engine and method of operating the engine which enables the engine to transfer between a spark ignition mode and a premixed charge compression ignition mode by adjusting the timing of the closing of the intake valves via a variable valve timing system.